This proposal will trace one of the neural pathways that has been implicated in the ability to selectively listen to a single sound source, or voice, in the presence of competing background noise. In most mammals, including man, these descending or efferent pathways are diffuse and therefore difficult to study. This project will use a mammal, the mustache bat, in which target cells of the descending system are unusually collected or focused. In this bat, the descending fibers from the superior olivary complex concentrate upon one cell group, the marginal zone, found in a lower auditory center, the anteroventral cochlear nucleus. Cells and fibers within the marginal zone are uniformly acetylcholinesterase positive. In other mammals, cholinergic efferents to cochlear nucleus have a strong influence upon unit responses to acoustic stimuli. There is evidence that these efferents may function as part of a system to enhance signal detection in noise. This study will use two standard anatomical tracing techniques, horseradish peroxidase and tritium autoradiography, to follow neural fibers either toward or away from their cell bodies. In all experiments, to correlate the anatomy and physiology of these descending pathways, neuronal tracers will be deposited at neural sites that are first located and characterized by electrophysiological recording of unit response properties. Deposits made from recording micropipettes will be used to determine: 1) the specific cells and cell types of the superior olive which project to the cochlear nucleus, and 2) the relation between these descending pathways and ascending pathways from cochlear nucleus. These studies will provide the necessary anatomical basis for long-term physiological and behavioral study of interactions between the neuronal population of the cochlear nucleus and neurons of the superior olivary complex implicated in efferent system activity. Better understanding of the basic organization and function of the descending auditory system may offer clues for increasing selective auditory perception with progressive hearing loss.